How to improve thermal efficiency of high temperature devices, such as gas turbines and jet engines, is an important problem for many reasons including the need to reduce environmental impacts. An effective way of increasing thermal efficiency is to increase service temperatures.
Currently, a turbine inlet temperature of about 1300° C. is standard in a gas turbine. On the other hand, turbine components applicable to temperatures around 1700° C. are becoming commercially practical. Also, for gas turbine components such as turbine blades, Ni-based alloys of high heat-resistant superalloys are often used.
Meanwhile, high-strength Ni-based alloys applied to these gas turbines, jet engines, etc. derive their high mechanical strength from precipitating a γ′ phase (gamma prime phase, Ni3Al) therein. A γ′ phase is coherent with a γ phase in crystalline lattice, and the γ′ phase coherently precipitated in the γ phase (hereinafter referred to as a “coherent γ′ phase”) contributes greatly to the improvement in mechanical strength. In other words, the mechanical strength of Ni-based alloy members used in gas turbines, etc. can be improved by increasing the amount of the precipitated γ′ phase. However, such high-strength Ni-based alloy members with a high content of the precipitated γ′ phase have extremely poor cold workability due to their high hardness, and therefore high-strength Ni-based alloy members are not usually cold-worked.
For example, turbine blades mentioned above are produced of Ni-based alloys by precision forging, in which a γ′ phase precipitate is present at a ratio of 36 to 60 volume %, and cold working is not carried out in the production process due to their high hardness.
On the other hand, as for combustor components produced by cold working, hardness can be reduced by using Ni-based alloys in which a γ′ phase precipitate is present at a controlled ratio of 30 volume % or lower, thereby making cold working possible. However, such combustor components and other articles that can be cold-worked have lower mechanical strength than turbine blades or the like produced of Ni-based alloys including a γ′ phase precipitate at a ratio of 36 to 60 volume %. And, such Ni-based alloys including a γ′ phase precipitate of 30 volume % or lower are not adequate to fully satisfy requirements for the capability to tolerate increasingly high temperatures, as mentioned above.
As seen from the above, what is strongly needed in the art is to develop an Ni-based alloy member that is produced of an Ni-based alloy including a γ′ phase precipitate of 36 to 60 volume % and having a high durable temperature and that further has good cold workability. Also, a method for producing such a member is required.
Patent Literature 1 discloses a method for making an Ni-based superalloy article having a controlled grain size from a forging preform. In Patent Literature 1, there is described a controlling method of a grain size of an Ni-based superalloy, comprising the steps of hot die forging as the initial forging operations and isothermal forging as the subsequent forging operations. With this controlling method, a uniform grain size of approximately ASTM 6 to 8 can be achieved by carrying out hot die forging for the initial upset followed by isothermal forging and, if necessary, subsolvus annealing to provide a microstructure suitable for supersolvus heat treatment. It also describes that the hot die forging causes partial or complete recrystallization of the microstructure, which facilitates superplastic deformation in the subsequent isothermal forging operations. Moreover, Examples disclosed in Patent Literature 1 include a description about grain sizes when heat treatment is applied at 1850° F., 1900° F., and 1925° F.